Evidence of the fractional quantum spin Hall effect in moiré MoTe₂.

Abstract

Quantum spin Hall (QSH) insulators are two-dimensional electronic materials that have a bulk band gap similar to an ordinary insulator but have topologically protected pairs of edge modes of opposite chiralities^1-6. So far, experimental studies have found only integer QSH insulators with counter-propagating up-spins and down-spins at each edge leading to a quantized conductance G₀ = e²/h (with e and h denoting the electron charge and Planck's constant, respectively)^7-14. Here we report transport evidence of a fractional QSH insulator in 2.1° twisted bilayer MoTe₂, which supports spin-S_z conservation and flat spin-contrasting Chern bands^15,16. At filling factor ν = 3 of the moiré valence bands, each edge contributes a conductance <math xmlns="http://www.w3.org/1998/Math/MathML"> <mrow> <mfrac><mrow><mn>3</mn></mrow> <mrow><mn>2</mn></mrow> </mfrac> <msub><mrow><mi>G</mi></mrow> <mrow><mn>0</mn></mrow> </msub> </mrow> </math> with zero anomalous Hall conductivity. The state is probably a time-reversal pair of the even-denominator 3/2-fractional Chern insulators. Furthermore, at ν = 2, 4 and 6, we observe a single, double and triple QSH insulator with each edge contributing a conductance G₀, 2G₀ and 3G₀, respectively. Our results open up the possibility of realizing time-reversal symmetric non-abelian anyons and other unexpected topological phases in highly tunable moiré materials^17-19.